The present invention relates to the fabrication of integrated circuits. More particularly, the invention provides a method and apparatus for directing process gas through a processing chamber.
High density integrated circuits, commonly termed VLSI devices, are typically formed on semiconductor wafers by subjecting the wafers to a number of deposition, masking, doping and etching processes. The wafers are placed onto a pedestal or susceptor within a process chamber and process gas(es) are delivered into the chamber onto the wafer to perform the various deposition and etching steps. For example, one typical process involves delivering SiH.sub.4 and N.sub.2 into the process chamber while applying RF energy to form a plasma for depositing silicon nitride on the wafer. Once the appropriate layer of silicon nitride is deposited onto the wafer, the remaining plasma and gas residue are withdrawn from the process chamber.
An important consideration in semiconductor processing is the application of process gases in a uniform and controlled manner across the wafer's entire surface. This consideration is particularly important in the fabrication of large scale integration (LSI) and very large scale integration (VLSI) devices since a large number of processing steps are generally used in sequence. To achieve uniform processing, existing systems typically introduce gases into the process chamber through a manifold plate or showerhead having a plurality of small openings for distributing the gas in a substantially uniform manner over the wafer. After the etching or deposition step has been completed, the plasma and gas residue are withdrawn from the process chamber by a suitable vacuum source, such as a pump. To facilitate uniform pumping dynamics, the process fluids will typically be discharged through a number of circumferentially spaced gas inlets in a pumping plate surrounding the susceptor. The fluid is then delivered through outlets in the pumping plate into a pumping channel disposed radially outward from and underneath the susceptor.
Many existing systems for directing process gas and plasma through semiconductor process chambers suffer from a number of drawbacks. For example, one known system comprises an annular pumping plate surrounding the wafer and having a number of fluid inlets circumferentially spaced about the perimeter of the plate. Gas holes extend straight through the pumping plate in a perpendicular direction relative to the wafer, thereby allowing the process gas to flow downward directly into the underlying pumping channel. Since the gas inlets are located at the perimeter of the plate, however, the gas must flow radially outward through the process chamber from the wafer to the perimeter of the plate before it exits the chamber through the gas inlets. This relatively long chamber flow path tends to increase the residence time of the gases within the chamber, thereby disrupting the uniform discharge of the gases from the wafer. In addition, a relatively larger chamber volume is generally required to accommodate the long chamber gas flow path, which increases the time required to remove contaminants from the chamber after the processing steps have been completed.
In another known system, the pumping plate comprises a plurality of gas holes extending perpendicular to the wafer and having circumferentially spaced inlets located adjacent to the wafer. The plate defines curved interior flow channels for allowing the gas to flow in a radially outward direction from the inlets to gas outlets located above the pumping channel. Since the inlets of the gas holes are located near the wafer, the pumping plate can be designed to extend into a portion of the process chamber, thereby decreasing the volume of the chamber. One disadvantage with this plate, however, is that the gases and plasma may become trapped within the curved interior flow channels, thereby clogging these channels and interfering with the pumping dynamics of the system. The curved interior flow channels also tend to increase the residence time of the gases within the process chamber because they do not provide a direct flow path from the chamber to the pumping channel. In addition, these interior channels increase the cost to manufacture the pumping plate and they can be relatively difficult to clean after the processing steps have been completed.
What is needed in the semiconductor manufacturing industry, therefore, are methods and apparatus for directing fluid through a process chamber. These methods and apparatus should be capable of discharging gases and plasma from the chamber in a controlled and uniform manner to minimize the creation of defects in the integrated circuitry being formed on the semiconductor wafer. In addition, these methods and apparatus should be designed to minimize the volume of the process chamber to reduce the residence time of the gases within the chamber and the time required to clean the chamber after processing has been completed.